The conventional way to correct defective vision of the human eye is a pair of spectacles. However, meanwhile use is also increasingly being made of refractive surgery which effects a correction of defective vision by altering the cornea of the eye. The aim of the operating methods is in this case to alter the cornea selectively in order to influence the refraction of light. Different surgical methods are known for this purpose. Most widespread at present is what is known as laser-assisted in situ keratomileusis, which is also abbreviated to LASIK. In this case, a corneal lamella is firstly detached from the surface of the cornea on one side and folded to the side. This lamella can be detached by means of a mechanical microkeratome or else by means of what is known as a laser keratome, such as is sold by Intralase Corp. Irvine, USA, for example. Once the lamella has been detached and folded to the side, the LASIK operation provides the application of an excimer laser which removes by ablation the corneal tissue exposed in this way. Once the volume positioned in the cornea has been vaporised in this way, the corneal lamella is folded back to its original site.
The application of a laser keratome to expose the lamella (also referred to as a flap) is advantageous, as this reduces the risk of infection and increases the quality of the cut. In particular, the lamella can be produced at a very much more constant thickness. The cut is also potentially smoother; this reduces subsequent optical disturbances caused by this interface which remains even after the operation.
When generating a cut face in the cornea by laser radiation, conventionally pulsed laser radiation is introduced into the tissue, the pulse length being generally less than 1 ps. As a result, the power density, which is necessary for triggering an optical breakthrough, for the respective pulse is confined to a small spatial area. In this regard, U.S. Pat. No. 5,984,916 clearly discloses that the spatial region of the optical breakthrough (in this case the interaction generated) highly depends on the pulse duration. A high focusing of the laser beam in combination with the aforementioned short pulses thus allows the optical breakthrough to be employed with pinpoint accuracy in the cornea. For generating the cut, a series of optical breakthroughs is generated at predetermined points in such a way that the cut surface is formed as a result. In the aforementioned laser keratome, the cut surface forms the lamella which is to be folded down before the use of the laser ablation.
In the conventional LASIK method, exposed corneal tissue is vaporised; this is also referred to as “grinding” of the cornea by means of laser radiation. The removal of volume that is necessary for correcting defective vision is in this case set for each surface element of the exposed cornea by the number of laser pulses and the energy thereof. Different amounts of material are removed in accordance with the number and energy of the laser pulses.